Liquid crystal display device

ABSTRACT

In a liquid crystal display device, in each unit pixel which is formed as a region surrounded by scanning signal lines and date signal lines includes pixel electrode to which signals of the date signal line is electrically supplied through a thin film transistor and common electrode which are electrically connected with the common signal line, and common signal electrode is arranged to be superposed on the common signal line by way of an insulation layer, and the pixel electrode is electrically connected with a source electrode of the thin film transistor via a through hole which penetrates the insulation layer, and the common electrode is formed such that the common electrode extend in the inside of the unit pixel while covering the common signal line.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/756,606, filed Jan. 14, 2004 now U.S. Pat. No. 7,098,982, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display device inwhich a technique is employed for suppressing the generation ofundesired electric fields that can be the cause of image retention,which tends to degrade the image quality of the display device.

In a liquid crystal display device, a liquid crystal layer formed ofliquid crystal molecules is sandwiched between two sheets of insulatingsubstrates, preferably made of glass, and, at the same time, at least apair of electrodes for applying an electric field to the liquid crystallayer are provided to either one or both of the substrates. In an IPS(in-plane switching) type of liquid crystal display device, all of theabove-mentioned electrodes for applying an electric field to the liquidcrystal layer are formed on one substrate, and the pixels are turned onand off (that is, switching is effected) so as to selectively form anelectric field having components that are parallel to the substratesurface with respect to the liquid crystal layer in selected pixels.

SUMMARY OF THE INVENTION

FIGS. 14A and 14B are diagrammatic views of the vicinity of one pixel ofa liquid crystal display device in which an IPS method is employed,wherein FIG. 14A is a plan view and FIG. 14B is a cross sectional viewtake along a line A-A′ in FIG. 14A. To an inner surface of one substrate(first substrate, usually referred to as the thin film transistorsubstrate, since it constitutes a substrate on which thin filmtransistors are formed, as will be explained hereinafter) SUB1 of a pairof substrates which constitute the liquid crystal display device, thefollowing pixel structure is provided. Then, a second substrate SUB2(not shown in the drawing) on which color filters and the like areformed is laminated to the first substrate SUB1, and a liquid crystallayer is sandwiched and sealed in a gap defined between both substrates.

On an inner surface of one pixel (also referred to as a unit pixelhereinafter), a plurality of scanning signal lines GL extend in thefirst direction (referred to as X direction hereinafter) and arearranged in parallel in the second direction (referred to as Y directionhereinafter) which intersects the X direction; a plurality of datesignal lines DL extend in the Y direction and are arranged in parallelin the X direction; common signal lines (also referred to as commonstorage lines) CL are disposed close to the scanning signal lines GL andextend in the X direction, while being arranged in parallel in the Ydirection; a plurality of thin film transistors TFT are arranged atintersecting portions of the scanning signal lines GL and the datesignal lines DL; pixel electrodes PX are driven by the thin filmtransistors TFT; and common electrodes CT are connected to the commonsignal line CL and are alternately arranged with respect to the pixelelectrodes PX such that the common electrode CT is arranged close to thepixel electrode PX in the X direction. Thus, unit pixels are formed inregions which are surrounded by the scanning signal lines GL and thedata signal lines DL.

As shown in FIG. 14B, which shows a cross section taken along a lineA-A′ in FIG. 14A, the common signal line CL is formed on the firstsubstrate SUB1, and a data signal line DL and a source electrode SD ofthe thin film transistor TFT are formed on the common signal line CL byway of a gate insulation layer GI. These electrodes or wiring arecovered with a laminated film which is constituted of an inorganicinsulation layer PAS and an organic insulation layer OPAS, and the pixelelectrodes PX and the common electrodes CT are formed on the laminatedfilm. Although an orientation film which is brought into contact with aliquid crystal layer (not shown in the drawing) is formed over the pixelelectrodes PX and the common electrodes CT, these elements are omittedfrom the drawing.

The pixel electrodes PX and the common electrodes CT are alternatelyarranged close to each other in a comb-teeth shape. As shown in FIG.14B, the pixel electrodes PX are connected to the source electrode SD,which constitutes an output electrode of the thin film transistor TFT,via a through hole SH. As shown in FIG. 14A, the source electrode SD issuperposed on the common signal line CL, and a portion which forms thethrough hole SH projects into the inside of the unit pixel region in astep-like manner. Here, the reference symbol OR in FIG. 14A of thedrawing indicates the direction of orientation control performanceapplied to the orientation film (the so-called rubbing direction).Further, an area indicated virtually between dashed lines represents alight blocking film (a black matrix) BM, which is usually formed on thesecond substrate. Here, the indication of the light blocking film BM isused in respective drawings to be referred to hereinafter in the samemanner.

With respect to a liquid crystal display device having such aconstitution, in the periphery of the unit pixel, particularly betweenthe common signal line CL and the pixel electrode PX, or the commonelectrode in the vicinity of the thin film transistor TFT, an undesiredelectric field is generated, and this electric field gives rise to adrawback in that liquid crystal molecules of the liquid crystal layerare switched without regard to the image data, thus generating aso-called image retention, whereby the image quality is degraded.

FIGS. 15A to 15C are diagrammatic views showing part of the structure inthe vicinity of the thin film transistor TFT in FIG. 14A, wherein FIG.15A is a plan view, FIG. 15B is a cross sectional view taken along aline B-B′ in FIG. 15A, and FIG. 15C is a cross sectional view takenalong a line C-C′ in FIG. 15A.

Reference symbol ZN in FIG. 15A indicates a region where the imageretention is liable to occur, reference symbol E in FIGS. 15B and 15Cindicates an electric field which becomes a cause of the occurrence ofimage retention, and reference symbol Ef in FIG. 15C indicates aparticularly strong electric field. That is, as shown in FIG. 15B andFIG. 15C, due to the electric field E that is generated between thecommon signal line CL and the pixel electrode PX, as well as due to theelectric field E generated between the source electrode SD and thecommon electrode CT, the liquid crystal molecules of the liquid crystallayer are switched on and off in an undesirable manner. Further, thestrong electric field Ef, having a large component in the directionwhich intersects the liquid crystal molecules, is generated between anedge of the source electrode SD and the common signal line CL, which aredisposed close to each other, and, hence, a large image retention isgenerated. As a result, the inventers of the present invention havefound that, due to switching of the liquid crystal molecules withoutregard to a normal switching operation of the unit pixel, irregularitiesare generated in the light (or reflection light), whereby the imagequality may be degraded.

Accordingly, it is an object of the present invention to provide aliquid crystal display device in which the problem of image retention isfurther suppressed compared to the above-mentioned liquid crystaldisplay device having the structure shown in FIG. 14, whereby an imagedisplay having a high image quality can be obtained.

To achieve such an object, the present invention adopts respectiveelectrode structures in which an electric field between a common signalline and a pixel electrode is blocked, an electric field between asource electrode of a thin film transistor and a common electrode isblocked or an electric field between an edge of the source electrode andthe common signal line is blocked.

The details of these electrode structures will become more apparent froma description of respective embodiments which represent various examplesof technical concepts of the invention, in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view and FIG. 1B is a sectional view taken along lineA-A′ in FIG. 1A, showing a unit pixel representing a first embodiment ofa liquid crystal display device according to the present invention;

FIG. 2A is a plan view and FIGS. 2B and 2C are sectional views takenalong lines B-B′ and C-C′, respectively, in FIG. 2A, showing thedetailed structure in the vicinity of the thin film transistor TFT inFIG. 1A;

FIG. 3A is a plan view and FIG. 3B is a sectional view taken along lineA-A′, showing the vicinity of a unit pixel in a second embodiment of aliquid crystal display device according to the present invention;

FIG. 4A is a plan view and FIGS. 4B and 4C are sectional views takenalong lines B-B′ and C-C′, respectively, in FIG. 4A, showing thedetailed structure in the vicinity of the thin film transistor TFT inFIG. 3A;

FIG. 5A is a plan view and FIG. 5B is a sectional view taken along lineA-A′ in FIG. 5A, showing the vicinity of a unit pixel in a thirdembodiment of a liquid crystal display device according to the presentinvention;

FIG. 6 is a plan view showing the detailed structure in the vicinity ofthe thin film transistor TFT in FIG. 5A;

FIG. 7A is a plan view and FIG. 7B is a sectional view taken along lineA-A′ in FIG. 7A, showing the vicinity of a unit pixel in a fourthembodiment of a liquid crystal display device according to the presentinvention;

FIG. 8 is a plan view showing the detailed structure in the vicinity ofthe thin film transistor TFT in FIG. 7A;

FIG. 9A is a plan view and FIG. 9B is a sectional view taken along lineA-A′ in FIG. 9A, showing the vicinity of a unit pixel in a fifthembodiment of a liquid crystal display device according to the presentinvention;

FIG. 10A is a plan view and FIG. 10B is a sectional view taken alongline B-B′ in FIG. 10A, showing the detailed structure in the vicinity ofthe thin film transistor TFT in FIG. 9A;

FIG. 11A is a plan view and FIG. 11B is a sectional view taken alongline A-A′ in FIG. 11A, showing the vicinity of a unit pixel in a sixthembodiment of a liquid crystal display device according to the presentinvention;

FIG. 12 is a plan view showing the detailed structure in the vicinity ofthe thin film transistor TFT in FIG. 11A;

FIG. 13 is a diagram showing an example of a unit pixel portion and aperipheral portion of the liquid crystal display device of the presentinvention;

FIG. 14A is a plan view and FIG. 14B is a sectional view taken alongline A-A′ in FIG. 14A, showing the vicinity of one pixel of a liquidcrystal display device which employs the IPS method; and

FIG. 15A is a plan view and FIGS. 15B and 15C are sectional views takenalone lines B-B′ and C-C′, respectively, in FIG. 15A, showing thedetailed structure in the vicinity of the thin film transistor TFT inFIG. 14A.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, several embodiments of the present invention will beexplained in detail in conjunction with the accompanying drawings.

FIG. 1A shows the constitution of a unit pixel in a first embodiment ofa liquid crystal display device according to the present invention, andFIG. 1B is a cross sectional view taken along a line A-A′ in FIG. 1A.Further, FIG. 2A shows the detailed structure in the vicinity of thethin film transistor TFT in FIG. 1A, FIG. 2B is a cross sectional viewtaken along a line B-B′ in FIG. 2A and FIG. 2C is a cross sectional viewtaken along a line C-C′ in FIG. 2A. In FIG. 1A and FIG. 2A, the samereference symbols are used to identify elements correspondinglyidentified in the above-mentioned FIG. 14A and FIG. 15A, and, hence, arepeated explanation of such elements will be omitted. The same goes forthe respective embodiments to be described hereinafter.

As seen in FIG. 1A and FIG. 2A, on an inner surface of a unit pixelwhich is formed on a first substrate SUB1, a plurality of scanningsignal lines GL extend in the X direction and are arranged in parallelin the Y direction, which intersects the X direction; a plurality ofdata signal lines DL extend in the Y direction and are arranged inparallel in the X direction; common signal lines CL are disposed closeto the scanning signal lines GL and extend in the X direction, whilebeing arranged in parallel in the Y direction; a plurality of thin filmtransistors TFT are arranged at intersecting portions of the scanningsignal lines GL and the data signal lines DL; pixel electrodes PX areprovided which are driven by the thin film transistors TFT; and commonelectrodes CT are connected to the common signal line CL and arealternately arranged with respect to the pixel electrodes PX, such thatthe common electrode CT is arranged close to the pixel electrode PX inthe X direction. Thus, the unit pixel is formed in a region surroundedby the scanning signal lines GL and the data signal lines DL.

The common electrodes CT are arranged in a superposed manner over thecommon signal line CL by way of an insulation layer, which isconstituted of an inorganic insulation layer PAS and an organicinsulation layer OPAS that is stacked on the inorganic insulation layerPAS. The same goes for all of the respective embodiments. However, inplace of the organic insulation layer OPAS, an inorganic insulationlayer may be used. Also in this case, the same goes for the respectiveembodiments to be described hereinafter. The pixel electrode PX isconnected to the source electrode SD of the thin film transistor TFT viathe through hole SH, which penetrates the insulation layer that isformed of the above-mentioned inorganic insulation layer PAS and organicinsulation layer OPAS. The common electrodes CT are formed such thatthey extend (project) into the inside of the unit pixel so as to coverthe common signal line CL, whereby an electric field E between thecommon signal line CL and the pixel electrode PX is blocked.

The pixel electrode PX has an end portion extending toward the inside ofthe unit pixel from the common electrode CT, while the source electrodeSD has a first projecting portion SD1 and a second projecting portionSD2 which project in a step-like manner in a direction which intersectswith the extending direction of the common electrodes CT. The step-likefirst projecting portion SD1 (source electrode side) is arranged betweenthe common signal line CL and the pixel electrode PX and, at the sametime, at a position which is sealed or blocked by the common electrodeCT. Further, the step-like second projecting portion SD2 (a portionwhich further projects toward the unit pixel side from the firstprojecting portion SD1) is connected to the pixel electrode PX via thethrough hole SH at a position where the second projecting portion SD2 issuperposed on the pixel electrode PX.

Further, assuming that the distance between an edge of the secondprojecting portion parallel to the Y direction and the pixel electrodePX neighboring in the X direction is “a”, the distance between the pixelelectrode PX and an end portion of the first projecting portion in the Ydirection is “b”, and the distance between the pixel electrode PX andthe common electrode CT neighboring in the Y direction is “c”, therelationships a>c and b>c are established. Further, the relationshipsa>c and b>c may be established. Due to the setting of suchrelationships, the electric field generated between the common signalline and the pixel electrode is blocked, or the electric field Egenerated between the source electrode SD of the thin film transistorTFT and the common electrode CT is blocked. Further, the electric fieldE generated between the edge of the source electrode SD and the commonsignal line CL is also blocked.

According to this embodiment, switching of the liquid crystal moleculeswithout regard to the normal switching operation of the unit pixel issuppressed, and, hence, no irregularities are generated with respect tothe transmitting light (or a reflection light) of the liquid crystallayer, whereby a high-quality image display can be obtained.

FIG. 3A shows the vicinity of a unit pixel in the second embodiment of aliquid crystal display device according to the present invention, andFIG. 3B is a cross sectional view taken along a line A-A′ in FIG. 3A.Further, FIG. 4A shows the vicinity of the thin film transistor TFT inFIG. 3A, while FIG. 4B is a cross sectional view taken along a line B-B′in FIG. 4A and FIG. 4C is a cross sectional view taken along a line C-C′in FIG. 4A.

As seen in FIG. 3A and FIG. 4A, on an inner surface of the unit pixelformed on a first substrate SUB1, a plurality of scanning signal linesGL extend in the X direction and are arranged in parallel in the Ydirection, which intersects the X direction; a plurality of data signallines DL extend in the Y direction and are arranged in parallel in the Xdirection; common signal lines CL are disposed close to the scanningsignal lines GL and extend in the X direction, while being arranged inparallel in the Y direction; a plurality of thin film transistors TFTare arranged at intersecting portions of the scanning signal lines GLand the data signal lines DL; pixel electrodes PX are provided which aredriven by the thin film transistors TFT; and common electrodes CT areconnected to the common signal line CL and are arranged with respect tothe pixel electrodes PX such that the common electrode CT is arrangedclose to the pixel electrode PX in the X direction. Thus, the unit pixelis formed in a region surrounded by the scanning signal lines GL and thedata signal lines DL.

The pixel electrode PX is arranged in a superposed manner over thecommon signal line CL by way of an insulation layer (an inorganicinsulation layer PAS and an organic insulation layer OPAS) and isconnected to a source electrode SD of the thin film transistor TFT via athrough hole SH, which penetrates the above-mentioned insulation layer.Further, a portion of the pixel electrode PX overhangs over the commonsignal line CL beyond the unit pixel, and an overhanging distal endforms an extending portion PXJ, which is spaced from a side of thecommon signal line CL at a side opposite to the pixel is electrode. Thecommon electrodes CT are formed such that they extend in the inside ofthe unit pixel covering the common signal line CL, except for a portionalong the extending portion PXJ of the pixel electrode PX. Due to such aconstitution, an electric field E between the common signal line CL andthe pixel electrode PX can be blocked.

Further, the extending portion PXJ of the pixel electrode PX has a widththat is larger than the width of the other portion of the pixelelectrode PX in the X direction, and the source electrode SD has aprojecting portion SD3 which projects in a step-like manner in thedirection which intersects the extending direction of the common signalline CL. The step-like projecting portion SD3 is arranged between thecommon signal line CL and the common electrode CT and is disposed at aposition which is blocked by the common electrode CT. The step-likeprojecting portion SD3 is arranged above the common signal line CL andat a position where the projecting portion SD3 is superposed on anextending portion PXJ of the pixel electrode PX and is connected to thepixel electrode PX via the through hole SH.

Then, assuming that the distance of the step-like projecting portion SD3of the source electrode SD from an end portion of the common signal lineCL in the Y direction is “a”, the distance in the X direction of theedge in the Y direction parallel to the common signal line CL contiguouswith the extending portion PXJ of the pixel electrode PX is “b”, and thedistance in the X direction between the edge in the Y direction of theoverhanging portion PXJ of the pixel electrode PX and the commonelectrode CT is “c”, the relationship a≧0 is established. Further, therelationship b≧c×2.0 is established. Further, the relationship a>0 andb>c×2.0 is established. Due to the setting of such relationships, theelectric field E between the common signal line CL and the pixelelectrode PX is blocked, or the electric field E generated between thesource electrode SD of the thin film transistor TFT and the commonelectrode CT is blocked. Further, the electric field E generated betweenthe edge of the source electrode SD and the common signal line CL isalso blocked.

According to this embodiment, a switching of the liquid crystalmolecules without regard to the normal switching operation of the unitpixel is suppressed, and, hence, no irregularities are generated withrespect to the transmitting light (or reflection light) of the liquidcrystal layer, whereby a high-quality image display can be obtained.

FIG. 5A shows the vicinity of a unit pixel in a third embodiment of aliquid crystal display device according to the present invention, andFIG. 5B is a cross sectional view taken along a line A-A′ in FIG. 5A.Further, FIG. 6A shows the detailed structure in the vicinity of thethin film transistor TFT in FIG. 5A.

As seen in FIG. 5A and FIG. 6A, on an inner surface of the unit pixelformed on a first substrate SUB1, a plurality of scanning signal linesGL extend in the X direction and are arranged in parallel in the Ydirection; a plurality of data signal lines DL extend in the Y directionand are arranged in parallel in the X direction; common signal lines CLare disposed close to the scanning signal lines GL and extend in the Xdirection, while being arranged in parallel in the Y direction; aplurality of thin film transistors TFT are arranged at intersectingportions of the scanning signal lines GL and the data signal lines DL;pixel electrodes PX are provided, which are driven by the thin filmtransistors TFT; and common electrodes CT are connected to the commonsignal line CL and are arranged with respect to the pixel electrodes PXsuch that the common electrode CT is arranged close to the pixelelectrode PX in the X direction. Thus, the unit pixel is formed in aregion surrounded by the scanning signal lines GL and the data signallines DL.

The pixel electrode PX is arranged in a superposed manner over thecommon signal line CL by way of an insulation layer (an inorganicinsulation layer PAS and an organic insulation layer OPAS) and isconnected to a source electrode SD of the thin film transistor TFT via athrough hole SH, which penetrates the insulation layer (the inorganicinsulation layer PAS and the organic insulation layer OPAS). Further, aportion of the pixel electrode PX includes an enlarged portion PXE whichbridges over the common signal line CL from the inside of the unitpixel. The common electrodes CT are formed such that they extend in theinside of the unit pixel covering the common signal line CL, except fora portion along the enlarged portion PXE of the pixel electrode PX. Dueto such a constitution, an electric field between the common signal lineCL and the pixel electrode PX can be blocked.

The enlarged portion PXE of the pixel electrode PX has an approximatelyrectangular shape, having two sides along the X direction of the unitpixel and another two sides along the Y direction, wherein the unitpixel side of the two sides along the X direction extends into and ispositioned in the inside of the unit pixel beyond the common signal lineCL, while the side opposite to the unit pixel of the two sides along theX direction is positioned inside an edge of the common signal line CL ata side opposite to the unit pixel and outside an edge of the sourceelectrode SD at a side opposite to the unit pixel.

Then, assuming the distance between the pixel electrode PX which extendsin the Y direction from the enlarged portion PXE to the inside of theunit pixel and the common electrode CT which is arranged close to thepixel electrode PX in the X direction is “a”, the distance between twosides of the enlarged portion PXE along the X direction and the commonelectrodes CT which are arranged close to the enlarged portion PXE is“b” and the distance between the unit pixel-PX-side out of the two sidesalong the X direction and the common signal line CL is “c”, arelationship a>b is established or a relationship b×0.5<c isestablished. Further, relationship relationships a>b and b×0.5<c areestablished. In this manner, by enlarging the pixel electrode PX, aregion which can block the common signal line CL can be enlarged,whereby the electric field E between the common signal line CL and thepixel electrode PX can be blocked, or the electric field between thesource electrode SD of the thin film transistor TFT and the commonelectrode CT can be blocked. Further, the electric field between theedge of the source electrode SD and the common signal line CL also canbe blocked.

FIG. 7A shows the vicinity of a unit pixel in a fourth embodiment of aliquid crystal display device according to the present invention, andFIG. 7B is a cross sectional view taken along a line A-A′ in FIG. 7A.Further, FIG. 8A shows the detailed structure in the vicinity of thethin film transistor TFT in FIG. 7A.

This embodiment is characterized in that portions of the common signalline CL have the width thereof narrowed at both sides of the projectingportion SD4 of the source electrode SD as compared to the embodiment 3,wherein a unit-pixel-side edge of the common signal line CL is retractedsuch that the distance between the unit-pixel-side edge of the commonsignal line CL and the unit-pixel-side side in the X direction of theenlarged portion PXE of the pixel electrode PX is increased. That is, inthe region of a portion of the source electrode SD2 which is coveredwith the enlarged portion PXE of the pixel electrode PX, in the Xdirection of the projecting portion SD4, which projects toward the unitpixel side and also projects in a step-like manner at the unit pixelside beyond the common signal line CL, a unit-pixel-side edge of thecommon signal line CL, which is arranged at the data signal line DL sideis retracted at a side opposite to the unit pixel and the relationshipof the distance a>distance b is established.

According to this embodiment, in addition to the advantageous effects ofthe third embodiment, a switching of the liquid crystal moleculeswithout regard to the normal switching operation of the unit pixel canbe further suppressed, and, hence, no irregularities are generated withrespect to the transmitting light (or reflection light) of the liquidcrystal layer, whereby a high-quality image display can be obtained.

FIG. 9A shows the vicinity of a unit pixel in a fifth embodiment of aliquid crystal display device according to the present invention, andFIG. 9B is a cross sectional view taken along a line A-A′ in FIG. 9A.Further, FIG. 10A shows the detailed structure in the vicinity of thethin film transistor TFT in FIG. 9A, while FIG. 10A is a plan view andFIG. 10B is a cross-sectional view taken along a line B-B′ in FIG. 10A.

As seen in FIG. 9A and FIG. 10A, on an inner surface of the unit pixelformed on a first substrate SUB1, a plurality of scanning signal linesGL extend in the X direction and are arranged in parallel in the Ydirection; a plurality of data signal lines DL extend in the Y directionand are arranged in parallel in the X direction; common signal lines CLare disposed close to the scanning signal lines GL and extend in the Xdirection, while being arranged in parallel in the Y direction; aplurality of thin film transistors TFT are arranged at intersectingportions of the scanning signal lines GL and the data signal lines DL;pixel electrodes PX are provided, which are driven by the thin filmtransistors TFT; and common electrodes CT are connected to the commonsignal line CL and are arranged with respect to the pixel electrodes PXsuch that the common electrode CT is arranged close to the pixelelectrode PX in the X direction. Thus, the unit pixel is formed in aregion surrounded by the scanning signal lines GL and the data signallines DL.

The pixel electrode PX is arranged in a superposed manner over thecommon signal line CL by way of an insulation layer (an inorganicinsulation layer PAS and an organic insulation layer OPAS) and isconnected to a source electrode SD of the thin film transistor TFT via athrough hole SH, which penetrates the insulation layer (the inorganicinsulation layer PAS and the organic insulation layer OPAS). Further, aportion of the pixel electrode PX includes an enlarged portion PXEbridges strides over the common signal line CL from the inside of theunit pixel.

The common electrodes CT are formed such that they extend in the insideof the unit pixel covering the common signal line CL, except for aportion along the enlarged portion PXE of the pixel electrode PX, and,hence, an electric field between the common signal line CL and the pixelelectrode PX can be blocked. The enlarged portion PXE of the pixelelectrode PX has two sides along the X direction of the unit pixel andtwo sides along the Y direction. The unit pixel side of the two sidesalong the X direction is positioned inside a unit-pixel-side edge (side)of the common signal line CL, while the side of two sides opposite tothe unit pixel PX is positioned outside the edge of the common signalline CL opposite to the unit pixel and outside the edge of the enlargedportion PXE of the pixel electrode PX opposite to the unit pixel.

A side of the source electrode SD of the thin film transistor TFTopposite to the unit pixel extends in the X direction close to the thinfilm transistor TFT to form one side of an extending portion SDE, whileanother side of the source electrode SD at the unit pixel side extendsin the X direction parallel to one side of the extending portion SDE atthe side opposite to the unit pixel from another of the two sides in theY direction close to the thin film transistor TFT. Then, assuming thatthe distance between another side of the extending portion SDE and theedge of the common signal line CL opposite to the unit pixel PX is “a”and the distance between the side opposite to the unit pixel PX ofanother two sides of the source electrode SD and the neighboring commonelectrode in the X direction is “b”, these distances are set as a≧0 andb≧0.

Accordingly, as shown in FIG. 10B, a portion where a fringe electricfield Ef, which constitutes a strong electric field formed between thesource electrode SD and the common signal line CL, can be completelyblocked by the pixel electrode PX, and, hence, switching of liquidcrystal molecules without regard to the normal switching operation ofthe unit pixel can be further suppressed, whereby no irregularities aregenerated with respect to the transmitting light (or reflection light)of the liquid crystal layer, whereby a high-quality image display can beobtained.

FIG. 11A shows the vicinity of a unit pixel in a sixth embodiment of aliquid crystal display device according to the present invention, andFIG. 11B is a cross sectional view taken along a line A-A′ in FIG. 11A.Further, FIG. 12A shows the detailed structure in the vicinity of thethin film transistor TFT in FIG. 11A.

As seen in FIG. 11A and FIG. 12A, on an inner surface of the unit pixelformed on a first substrate SUB1, a plurality of scanning signal linesGL extend in the X direction and are arranged in parallel in the Ydirection; a plurality of data signal lines DL extend in the Y directionand are arranged in parallel in the X direction; common signal lines CLare disposed close to the scanning signal lines GL and extend in the Xdirection, while being arranged in parallel in the Y direction, aplurality of thin film transistors TFT are arranged at intersectingportions of the scanning signal lines GL and the data signal lines DL;pixel electrodes PX are provided, which are driven by the thin filmtransistors TFT; and common electrodes CT are connected to the commonsignal line CL and are arranged with respect to the pixel electrodes PXsuch that the common electrode CT is arranged close to the pixelelectrode PX in the X direction. Thus, the unit pixel is formed in aregion surrounded by the scanning signal lines GL and the data signallines DL.

The pixel electrode PX is arranged in a superposed manner over thecommon signal line CL by way of an insulation layer (an inorganicinsulation layer PAS and an organic insulation layer OPAS) and isconnected to a source electrode SD of the thin film transistor TFT via athrough hole SH, which penetrates the insulation layer (the inorganicinsulation layer PAS and the organic insulation layer OPAS).

Further, a portion of the pixel electrode PX includes an approximatelyrectangular enlarged portion PXE which bridges over the common signalline CL from the inside of the unit pixel, while the source electrode SDincludes a projecting portion SD4 which projects toward the unit pixelside at a portion which is covered with the enlarged portion PXE of thepixel electrode PX and projects toward the unit pixel side in astep-like manner beyond the common signal line.

The common electrode CT is formed such that it extends (overhangs) inthe inside of the unit pixel, while covering the common signal line CL,except for a portion of the common signal line CL, along the enlargedportion PXE of the pixel electrode PX so as to block an electric fieldbetween the common signal line CL and the pixel electrode PX. At thesame time, the common electrode CT includes common electrode enlargedportions CTE, which are enlarged to cover the common signal lines CLcorresponding to the enlarged portion PXE of the pixel electrode PX atportions of the thin film transistor TFT side, while having sides whichextend at an angle θ in the X direction with respect to the Y directionand sides which extend in the Y direction. Here, an angle at which thepixel electrode PX in the unit pixel rises is also set to θ.

The enlarged portions PXE of the pixel electrode PX include two sidesalong the X direction of the unit pixel and two other sides which extendin the Y direction of the unit pixel, wherein the unit pixel side of thetwo sides along the X direction is positioned more inside of the unitpixel than the common signal line CL and the other side of the two sidesopposite to the unit pixel is positioned more inside than an edge thecommon signal line CL opposite to the unit pixel.

Here, by setting the angle θ to 90 °≦θ<180°, the pixel electrode PX andthe common electrode CT are enlarged. Further, the relationship betweenthe distance “a” between the side of the common electrode enlargedportion CTE which extends in the X direction and the step-like edgeextending along the first direction, the distance “b” between theunit-pixel-side edge of the common signal line CL and the edge along theX direction of the projecting portion SD4 of the source electrode SDwhich projects in a step-like manner, and the distance “c” between thepixel electrode PX and the edge in the Y direction of the commonelectrode enlarged portion CTE is set to a>b, and is preferably set to(a−b)>c. Due to such a constitution, an electric field from the commonsignal line CL can be surely blocked, and, hence, the image retentionareas are reduced, whereby the effective numerical aperture can beincreased.

Here, in the above-mentioned respective embodiments, by setting thedistance between the pixel electrode PX and the source electrode SD inFIG. 1A to be smaller than the distance between the pixel electrode PXand the common electrode CT, which are arranged to close each other, forexample, a DC component which remains between the electrodes can bereduced; and, hence, the image retention can be suppressed, and, at thesame time, the effective numerical aperture is enhanced. Here, as shownin FIG. 14A, by forming a black matrix BM such that the black matrix BMcovers the longitudinal end portions of the pixel electrodes PX and thecommon electrodes CT inside of the unit pixel, a degradation of imagequality attributed to image retention can be further suppressed.

Further, in the above-mentioned respective embodiments, by furtherarranging the common signal lines CL along both sides of the scanningsignal line GL, it is possible to more effectively block the leaking ofthe electric field from the scanning signal line GL. Here, the endportions of the common electrodes CT are formed such that the endportions are retracted from the common electrodes CT inside of the unitpixel.

FIG. 13 is a schematic cross-sectional view showing one example of theunit pixel portion and the peripheral portion of the liquid crystaldisplay device of the present invention. In the drawing, SUB1 indicatesthe first substrate which has been identified in conjunction with therespective embodiments. A first orientation film ORI1 is applied to anuppermost layer on the first substrate SUB1, which is brought intocontact with the liquid crystal layer LC, and rubbing treatment isapplied to the first orientation film. Then, a first polarizer POL1 isformed on an outer surface of the substrate SUB2. Here, the samereference symbols are used to identify identical functional portions inthe respective embodiments, wherein PSV indicates a protective film andthe constitution of the first substrate SUB1 is simplified.

Further, SUB2 indicates the second substrate, which constitutes thecounter substrate, wherein the second substrate SUB2 includes colorfilters CF, which are defined by the black matrix BM, and an overcoatlayer OC, which is formed above the color filters CF. Further, a secondorientation film ORl2, which is brought into contact with the liquidcrystal layer LC, is applied to the overcoat layer OC, and rubbingtreatment is applied to the second orientation film ORl2. A secondpolarizer POL2 is mounted on an outer surface (a viewing side) of thesecond substrate SUB2. A sealing material SL seals between peripheralportions of the first substrate SUB1 and the second substrate SUB2.

The electric field EP which effects switching of the unit pixel (turningon/off of the unit pixel) is formed between the pixel electrode PX andthe common electrode CT in the direction parallel to the respectivesubstrate surfaces.

As has been explained heretofore, according to the present invention,the generation of an undesired electric field between the common signallines and the pixel electrodes and the common electrodes in theperiphery of the unit pixel, particularly in the vicinity of the thinfilm transistor, can be suppressed, and, hence, the occurrence of imageretention can be reduced, whereby it is possible to provide a liquidcrystal display device which is capable of displaying high qualityimages.

1. A liquid crystal display device comprising first and secondsubstrates, a liquid crystal layer interposed between the first and thesecond substrates, a plurality of scanning signal lines and a pluralityof data signal lines which are formed on the first substrate, and commonsignal lines which are arranged close to the scanning signal lines,wherein each unit pixel which is formed as a region surrounded by thescanning signal lines and the data signal lines includes a pixelelectrode to which signals of the data signal line are electricallysupplied through a thin film transistor and a common electrode which iselectrically connected with the common signal line, wherein the commonelectrode extends in a direction of elongation of the data signal linesand includes an extension portion to be superposed on the common signalline by way of an insulation layer, wherein a width of the extensionportion is wider than a width of the common electrode of a portion whichis not overlapped with the common signal line and the scanning signalline, wherein the pixel electrode is electrically connected with asource electrode of the thin film transistor via a through hole whichpenetrates the insulation layer, wherein a portion of the pixelelectrode includes a connecting portion which is elongated in adirection in parallel with a direction of elongation of the commonsignal line, and a portion of the connecting portion of the pixelelectrode is formed to overlap with the common signal line, wherein theextension portion of the common electrode and the elongated connectingportion of the pixel electrode have overlapping portions with the commonsignal line and extend in the same direction with respect to theextending direction of the scanning signal line, and wherein, assuming adistance between an end portion of the extension portion of the commonelectrode and an end portion of the source electrode on the commonsignal line as “a”, a distance between an end portion of the sourceelectrode above the common signal line and an end portion of the commonsignal line as “b”, and a distance between the extension portion of thecommon electrode and the elongated connecting portion of the pixelelectrode on the common signal line as “c”, the relationship (a−b)>c isestablished.
 2. A liquid crystal display device according to claim 1,wherein an angle θ which is made by the common electrode and theextension portion of the common electrode is set to 90°≦θ<180°.
 3. Aliquid crystal display device according to claim 1, wherein an angle θwhich is made by the pixel electrode and the overlapping portion of thepixel electrode is set to 90°≦θ<180°.
 4. A liquid crystal display deviceaccording to claim 1, wherein the angle which is made by the pixelelectrode and the overlapping portion of the pixel electrode issubstantially equal to the angle which is made by the common electrodeand the extension portion of the common electrode.
 5. A liquid crystaldisplay device according to claim 1, wherein the source electrode islonger than the elongated connecting portion of the pixel electrode in adirection extending in parallel with the scanning signal line, and thesource electrode is formed over a part of the extension portion of thecommon electrode and a part of the common signal line.